Re: [RFC 0/4] Virtio uses DMA API for all devices

[Anshuman Khandual]

On 08/05/2018 05:54 AM, Michael S. Tsirkin wrote:
> On Fri, Aug 03, 2018 at 08:21:26PM -0500, Benjamin Herrenschmidt wrote:
>> On Fri, 2018-08-03 at 22:08 +0300, Michael S. Tsirkin wrote:
>>>>>> Please go through these patches and review whether this approach broadly
>>>>>> makes sense. I will appreciate suggestions, inputs, comments regarding
>>>>>> the patches or the approach in general. Thank you.
>>>>>
>>>>> Jason did some work on profiling this. Unfortunately he reports
>>>>> about 4% extra overhead from this switch on x86 with no vIOMMU.
>>>>
>>>> The test is rather simple, just run pktgen (pktgen_sample01_simple.sh) in
>>>> guest and measure PPS on tap on host.
>>>>
>>>> Thanks
>>>
>>> Could you supply host configuration involved please?
>>
>> I wonder how much of that could be caused by Spectre mitigations
>> blowing up indirect function calls...
>>
>> Cheers,
>> Ben.
> 
> I won't be surprised. If yes I suggested a way to mitigate the overhead.

Did we get better results (lower regression due to indirect calls) with
the suggested mitigation ? Just curious.